The direct conversion of gases to liquid fuels and chemicals is highly sought, especially at low-scale to more effectively utilize remote natural gas resources. Direct conversion has proven to be a very difficult problem, because gases such as methane are highly unreactive, though tend to over-oxidize once activated. Liquid fuels such as methanol are primarily produced today by the steam reformation of methane at high temperatures and pressures to synthesis gas (carbon monoxide plus hydrogen), followed by catalytic conversion at intermediate temperatures and high pressures. Though highly selective, this technology is not well suited for low production wellheads.
The development of methods to directly convert low cost gases to valuable liquid fuels and chemicals that are easily transportable is highly sought. The need is especially great at low-scale, such as at low-volume wellheads in oilfields where considerable quantities of valuable natural gas must be flared. A compact, portable device that requires minimal maintenance and is easily installed and operated at remote sites would be especially valuable to better utilize available resources. Methanol is a particularly valued product, with a high volumetric energy density and easily transported.
Despite substantial research efforts directed at the direct conversion of low cost gases to higher value fuels for the past few decades, no viable commercial process has emerged while avoiding conditions that could lead to deep fuel oxidation.
Selective oxidation presents at least two significant challenges: 1) how to activate C—H bonds using the lowest amount of energy possible to initiate the transformation and 2) how to harness the oxidation reaction driven by thermodynamics to selectively targeted compounds while in the presence of oxygen. For example, the strength of the C—H bond in methane is 440 kJ/mol, while that in methanol is 389 kJ/mol). Thus, the C—H bond in methane is stronger than in the possible products meaning that the products will be more reactive than methane. Table 1 summarizes direct methane to methanol technologies and their status.
TABLE 1Methane to Methanol Technologies.RouteReaction typeCatalystIssuesHighRadical, gas-phasenoneLow temperaturehomogeneousselectivityIntermediateHeterogeneous metal oxides (VOx,Low temperaturecatalyticFeOx, MoOx), metalselectivityalloys (Cu/Zn)and yieldLowHomogeneous Pt(II) &Pt(IV)Complex andtemperaturecatalytic in complexes;costlysolutionorganometalic complexAmbientHeterogeneous enzymesLow conditionscatalyticconversions,not practical
Considering the reaction conditions (elevated temperature and strong oxidant), the selectivity is a real challenge.